Relative hydraulic conductivity and effective saturation from Earth’s field nuclear magnetic resonance – a method for assessing the vadose zone

Abstract

Beside the water content, petrophysical nuclear magnetic resonance (NMR) techniques in the lab and in boreholes as well as in the field, provide estimates of the hydraulic conductivity of water saturated sediments and rocks. In the vadose zone, the hydraulic conductivity is a function of the water saturation. Regarding the characterization of the vadose zone, the magnetic resonance sounding (MRS) method is expected to have great potential. However, so far, the petrophysical relationship of the hydraulic properties under partial saturation conditions and the NMR parameters in the Earth’s magnetic field is not fully understood. In this study, laboratory NMR experiments in the Earth’s field (EFNMR) are performed in comparison to conventional high field NMR (HFNMR). Sand‐filled columns were used to generate partially saturated conditions by simulating capillary fringes (grain sizes from fine to coarse). We investigate the ability of both NMR techniques to determine the residual water content and the dependency of the NMR relaxation times on the water saturation degree. We note that EFNMR measurements tend to underestimate the residual water content due to long measurement dead times. Furthermore, it shows that the HFNMR relaxation time T2, as a function of the saturation, behaves according to the Brooks‐Corey model that describes the water retention function and thus allows for the prediction of the relative hydraulic conductivity Krel,. The EFNMR relaxation time T2* as a function of the saturation degree differs from the Brooks‐Corey expectation due to the influence of the dephasing relaxation rate that is, in general, responsible for the difference of T2 and T2*. We assume that the dephasing relaxation rate itself, when induced by internal magnetic field gradients, depends on the water saturation. We introduce a model that accounts for this dependency with a weighting factor for the dephasing relaxation rate, given as a power law of the saturation degree. The model enables the description of T2* as a function of the water saturation and thus provides the estimation of Krel from T2*. We compare the NMR based Krel predictions with the Krel functions estimated from gravity induced outflow experiments at the columns. The results are in agreement within half a decade for every sand sample of the study. In principle, the suggested approach can be applied for estimating Krel in situ by MRS measurements in the vadose zone. We discuss the potential and limitations of this approach for MRS.

A laboratory study to determine the effect of pore size, surface relaxivity, and saturation on NMR T2 relaxation measurements

“…While the literature on NMR measurements of water‐saturated materials is extensive, there are only a limited number of studies that focus on the interpretation of NMR measurements for applications in the vadose zone. Such studies have shown that the ‐distribution, via its relation to the PSD, can be used to estimate the water retention curve (e.g., Costabel and Yaramanci ; Costabel and Yaramanci ; Jaeger et al . ); however, this interpretation requires that the NMR measurement be collected on a fully saturated sample and that relaxation occurs in the fast diffusion regime.…”

“…). Because of this relation, a number of studies have successfully estimated the unsaturated hydraulic conductivity and the water retention curve for unsaturated, unconsolidated materials (Bird and Preston ; Costabel and Yaramanci ; Costabel and Yaramanci ; Chen, Liaw, and Watson ; Ioannidis et al . ).…”

“…), for glass beads with mean diameters of 300 μm (Ioannidis et al . ), and for industrial sands with low iron(III) concentrations and mean grain diameters ranging from 0.06 mm to 2 mm (Costabel and Yaramanci ). While Ioannidis et al .…”

Estimation of water retention parameters from nuclear magnetic resonance relaxation time distributions

“…This approach was verified for glass beads by Ioannidis et al . [2006] and for sand of different grain sizes by Costabel and Yaramanci [2011b]. …”

Contribution of magnetic resonance soundings for characterizing water storage in the unsaturated zone of karst aquifers

“…We focused on the upper part of the soundings (depth <25 m and pulses <2500 A: ms) because of the best resolution, and these depths/pulses values are reached by all soundings. The relaxation time T Ã 2 ðe max Þ associated with maximum signal amplitude characterizes the lithological properties of the rock (including possible alteration and clay infilling) because the relaxation measured in a specific pore space always decreases with its saturation degree as shown in recent laboratory (e.g., Ioannidis et al, 2006;Boucher et al, 2011;Costabel and Yaramanci, 2011;Mohnke et al, 2015) and field studies (e.g., Costabel and Gunther, 2014;Walsh et al, 2014). On the other hand, θ Further investigation of the relationship between MRS water storage and flow factors, such as those used in vulnerability approaches is hindered by (1) the number of soundings performed, which does not permit statistical analyses such as principal component analysis, and (2) the uncertainty regarding MRS results, which is in part due to equivalency issues in the inversion of the MRS signal.…”